1. Field of the Invention
The present invention is a method of forming an inter-polysilicon oxide (IPO) layer of superior uniformity between two polysilicon layers (POLY1 and POLY2) in a microelectronics fabrication, such as, but not limited to, the fabrication of a charge coupled device (CCD), so that POLY2 does not undercut the IPO, stringer formation does not occur in POLY2, plasma damage is significantly reduced and fringe electric fields during the operation of the device can be controlled.
2. Description of the Related Art
Certain microelectronics devices, such as charge coupled devices (CCD), are formed by a process in which a silicon substrate is first covered by an oxide-nitride-oxide (ONO) layer (see C. Y. Chang and S. M. Sze, "ULSI Technology," McGraw-Hill Book Co., N.Y. 1996, p.168). An initial polysilicon (POLY1) layer or a topographic distribution of such layers is then deposited on this ONO substrate. This POLY1 layer is then covered by a polysilicon oxide layer, called the inter-polysilicon oxide layer (IPO), which is formed by wet thermal oxidation. As a final step in the fabrication, a second polysilicon layer (POLY2) is deposited over the IPO layer, Shih et al., U.S. Pat. No. 5,804,488, applies such a process to the formation of capacitors within the fabrication of microelectronic circuitry. Wuu et al, U.S. Pat. No. 5,480,814, discuss the formation of IPO layers over polysilicon gates in the context of the fabrication of certain self-aligned contact (SAC) structures. Liaw et al., in U.S. Pat. No. 5,866,499 discuss the formation of a triple-layer polysilicon structure separated by IPO layers. Yoo, in U.S. Pat. No. 5,470,779, discusses the formation of an SRAM structure in which an IPO is formed by plasma chemical vapor deposition (PE CVD) between a polysilicon layer and a semi-insulating polycrystalline silicon (SIPOS). The process by which the POLY2 layer is deposited over the IPO layer can cause the IPO layer to be undercut, leading to the formation of POLY2 stringers within the undercut area. In addition to the undercutting and stringer formation, the IPO layer may also be subject to non-uniformities as a result of the method of deposition
The present invention provides a method to form IPO layers in such a way as to eliminate the undercutting and stringer formation and increase the uniformity of the IPO.